One technique used to measure the phase and/or amplitude noise of a device at microwave frequencies is to interrogate the device using a reference signal. The device adds amplitude and/or phase noise to the reference signal to produce a modified signal. The modified signal is then compared to the reference signal using a double-balanced mixer, which produces a baseband signal representative of the phase or amplitude noise of the device, according to the relative phase between the reference signal and the modified signal input to the mixer.
A limitation of the sensitivity of this method is the noise of the mixer itself, which can dominate the baseband signal if the device is sufficiently low noise.
Improved methods and apparatus for measuring phase and/or amplitude characteristics were disclosed in Australian patent 695262 entitled “Phase noise detector” and Australian patent 736234 entitled “Interferometric signal processing apparatus”, the contents of both of which are incorporated herein by reference.
Both references utilise carrier suppression to enable amplification of the noise sidebands of the modified signal and to reduce the influence of the mixer noise on the baseband signal.
However, the mixer's noise, including DC voltage drift, still influences the baseband signal, albeit at a far lower level, and typically only at small frequency offsets from the microwave signal.
The previous discussion of the background art is intended to facilitate an understanding of the present invention only. It should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was part of the common general knowledge as at the priority date of the application.